1. Field of the Invention
The present invention relates to process rolls and, more particularly, is directed to rolls for processing uniformly flat products such as polymer sheet.
2. Description of the Invention Background
Rolls of many types and sizes are used for processing a variety of flat materials such as steel plate, paper, fabric, rubber, polymeric materials, etc. Typically, the material is passed through a series of stacked, temperature controlled rolls that serve to flatten the material into a continuous sheet or web having a desired thickness.
In many applications, it is critical for the sheet material to be formed with a uniform cross-sectional thickness. For example, polymer sheet used in the packaging industry for creating packages, commonly known as "bubble packs" is typically heated and vacuum formed into packages that have cavities that conform to the shape of the packaged articles. If the raw polymer sheet does not have a uniform cross-sectional thickness, it can ultimately lead to improperly formed parts when it is vacuum formed.
A flat sheet extrusion system of the type that is typically used to extrude molten polymer material into a flat sheet is depicted in FIG. 1. Such system, generally designated as 110, comprises a roll stand 112, a closed loop temperature control recirculating system 114, a cooling conveyor 116 and a pull roll assembly 118.
The roll stand 112 typically supports three driven "polishing" rolls (120, 130, 140) that are arranged in a stacked configuration as shown in FIGS. 1 and 2. In a roll arrangement known as a "downstack" the molten material, generally designated as 150, exits the die of an extruding machine (not shown) and is immediately pulled into the "nip" 121 between rolls 120 and 130 as shown in FIG. 2. The material 150 then passes around roll 130, enters the nip 131 between rolls 130 and 140, and passes around roll 140 to be received on cooling conveyor 116. In an "upstack" arrangement, the molten material 150 would enter the nip 131 between rolls 140 and 130, pass around roll 130 into the nip 121 between rolls 130 and 120 and pass around roll 120 to an adjacent cooling conveyor.
In most flat sheet extrusion arrangements, a pull roll assembly 118 is used to keep tension in the extruded flat sheet as it exits the final polishing roll. Slitting apparatus 119 is typically mounted on the cooling conveyor 116 for removing the outer edges of the sheet which, due to irregular cooling, are not uniformly flat. After the material 150 exits the pull rolls 118, it can be continuously rolled on a core (not shown) or, in applications wherein the sheet is relatively thick (i.e., greater than 1/8"), the material can be cut into sheets and stacked.
FIG. 3 illustrates a cross-sectional elevational view of rolls 120 and 130. As can be seen in that Figure, roll 120 has a shell 122 that is coaxially received on two journal portions 124. Roll 130 is similarly constructed and has a shell 132 that is coaxially received on two journals 134. The journals 134 of roll 130 are received in corresponding bearings 136 that are non-movably mounted to the roll support structure 112. Although journals 134 can freely rotate, the position of the roll 130 typically cannot be adjusted.
The journals 124 are supported in bearings 126 that are movably mounted to the support structure such that the positions of the bearings 126 can be adjusted in the directions depicted by arrows A' to achieve a desired amount of distance between shells 122 and 132 which ultimately will correspond to the thickness of the extruded sheet. As the sheet thickness is reduced, the molten polymer 150 typically tends to cool quicker because of its smaller mass as it contacts the first set of polishing rolls (120, 130). Such cooling causes the material 150 to stiffen and generate greater "separating" forces, collectively designated by arrow 152. These forces are generated by the material's resistance to deformation (i.e., the material's viscoelasticity). As can be seen from FIG. 3, the separating forces 152 tend to separate the shells (122, 132) as the polymer material 150 passes therebetween. To counteract the separating forces 152, the bearings 126 are typically loaded in the direction depicted by arrows 154 such that roll 120 is held under pressure relative to roll 130. However, such arrangement typically causes the rolls (120, 130) to deflect as shown in FIG. 3 which ultimately causes the extruded sheet 150 to acquire a "lens-like" cross-sectional shape.
When extruding polymeric sheet materials, the material must be cooled at a desired rate as it passes between and around the polishing rolls. Therefore, the polishing roll shells are typically hollow so that a liquid cooling medium can be pumped therein. It will be appreciated that, because the heat transfer takes place through the roll shell, relatively thin walled shells exhibit the best heat transfer capabilities.
It has been found that the deflection of such hollow-shell rolls can be reduced by increasing the shell wall thickness. However, rolls having relatively thick walled shells are more expensive to fabricate and exhibit poor heat transfer properties. Thus, prior rolls with high heat transfer requirements are not well suited for flat sheet extrusion operations wherein roll deflection must be minimized.
A variety of other rolls have been developed to minimize roll deflection. For example, U.S. Pat. No. 242,058 to Schurmann, discloses various roll embodiments that generally comprise a cylindrical body member that is coaxially received on or mounted to an axle. In one embodiment, the body member has a solid center portion and two axle or journal portions that extend outwardly from the solid center portion. The ends of the body portion are provided with recesses that extend around the corresponding axle portions to permit the ends of the body member to independently deflect relative to the adjacent axle portions. Schurmann discloses that his rolls can be used in connection with conventional rolls because they are able to adapt closely to shape of the conventional rolls. The Schurmann roll, however, lacks heat transfer means for controlling the temperature of the material that contacts the roll face.
Another roll is disclosed in U.S. Pat. No. 4,158,128 to Evdokimov et al. The roll disclosed in this patent comprises a core and an outer shell that is attached to the core. The shell is formed from two coaxial pipes that have corresponding center portions and end portions. The inner pipe is attached to the center portion of the core and the outer pipe is attached to the inner pipe in their middle portions and at their end portions. Such shell construction permits the outer pipe to flex in a wave-like manner when loads are applied to the face of the outer pipe and the core journals. This patent discloses that by proper selection of the degrees of stiffness of the core and the outer pipe, the supported portions of the outer pipe will lie in a straight line. The ends of the inner and outer pipes of the Evdokimov et al. roll may also be hermetically sealed such that a heating or cooling fluid may be admitted into the passage located between the inner and outer pipes. This temperature controlling medium is supplied to the passageway by pipelines that extend from one core journal into one end of the passageway. Because those pipelines support the ends of the shell relative to the core, they would also cause the shell to deflect in a wave like manner. Thus, this roll is not well suited for use with a conventional roll because a conventional roll face typically deflects in an arcuate manner. Also, due to its wave-like deflection pattern, the Evdokimov et al. roll would be ill-suited for applications wherein there can be little or no variation in the sheet thickness across the width of the sheet.
U.S. Pat. No. 4,823,450 to Ramisch et al. discloses a flex-compensated roll that comprises an outer roller shell that is supported on a core by a hydraulically operated support member that serves to support the outer roller shell relative to the core. Hydraulic fluid is pumped to the expandable and contractible support member located between the core and the outer roller shell to compensate for sagging of the outer roller shell. The temperature of the outer roller shell may be controlled by controlling the temperature of the hydraulic fluid. This roll arrangement, however, requires additional hydraulic fluid control equipment to be used.
Thus, there is a need for a roll that has high heat transfer capabilities that may be used in connection with a conventional roll to extrude flat sheet having a substantially uniform cross sectional thickness.
There is a further need for a roll having the above-mentioned characteristics that is lighter and more economical to fabricate than prior roll designs.